Cataracts are a common cause for poor vision. They are the leading cause of blindness with a prevalence of over 20 M worldwide. In addition, there are at least 100 M eyes with cataracts causing visual acuity of less than 6/60 in meters (or 20/200 in feet). Cataract extraction is the most commonly performed surgical procedure in the world with estimates of 10 million cases worldwide and 2 million cases being performed annually in North America. There are three types of cataract extraction: intracapsular, small incision cataract and phacoemulsification.
Currently, intracapsular surgery is commonly performed in developing countries where there are less resources. In this procedure both the opacified natural lens and the lens capsule are removed together.
In small incision cataract surgery and phacoemulsification the opacified natural lens is removed while leaving the elastic lens capsule intact to allow implantation and retention of the intraocular lens (IOL). One of the more critical surgical components is the capsulorhexis.
Capsulorhexis is the incision in the lens capsule to permit removal of the lens nucleus and cortex. The lens capsule is a transparent, homogeneous basement membrane that is made up of collagen-like protein. It has elastic properties without being composed of elastic fibers. The capsule has a smooth surface contour except at its equator where the zonules attach.
Ideally the capsulorhexis creates a symmetric circular incision, centered about the optical axis and is sized appropriately for the IOL and patient's condition. The mechanical integrity around the newly formed incision edge needs to be sufficient to withstand the forces experienced during cataract extraction and IOL implantation. By maintaining integrity of the remaining capsule, the IOL haptics are located in proximity of the capsule equator to allow location of the IOL. Postoperatively the newly formed capsule rim hardens and the opening contracts providing further strength and structural support for the IOL to prevent dislocation and misalignment. Because of this postoperative contraction it is important that the opening diameter and the IOL optic diameter are purposefully mismatched, otherwise the resultant pressures may cause the IOL to be dislocated.
The current standard of care for capsulorhexis is Continuous Curvilinear Capsulorhexis (CCC). The concept of the CCC is to provide a smooth continuous circular opening through the anterior lens capsule for phacoemulsification and insertion of the intraocular lens minimizing the risk of complications including errant tears and extensions. Currently, the capsulorhexis is performed manually utilizing forceps or a needle. The technique is dependent on applying a shear force and minimizing in-plane stretching forces to manually tear the incision.
The size of the capsulorhexis is determined by technique, the zonules strength and IOL optic diameter. Zonular strength is assessed prior to capsulorhexis. If the zonules appear strong then the capsulorhexis diameter should measure about 4-5 mm being about 0.5 to 1.0 mm smaller than the IOL optic diameter and centered on the optical axis. This provides overlap with the IOL and some margin for error. If the assessment shows a generally loose lens diaphragm then the capsulorhexis should be made about 5 to 7 mm in diameter being about 0.5 to 1.0 mm larger than the IOL optic diameter, again centered on the optical axis. If the zonular strength assessment shows asymmetric weakness the capsulorhexis should still be 5 to 7 mm in diameter being 0.5 to 1.0 mm larger than the IOL optic diameter, but located off-center away from the possible dehiscence. This would allow the IOL optic to be aligned and centered up with the capsulorhexis when the haptics are oriented toward the quadrant of zonular weakness. Larger diameter capsulorhexis are more difficult because the steepness of the capsule wall increases towards the capsule equator, thus it is harder to limit the shear forces without stretching the capsule, and the probability increases of a rhexis escape, i.e., an errant tear.
Errant tears are radial rips and extensions of the capsulorhexis towards the equator, moreover if a zonular attachment is encountered the tear is sent directly out to the capsular fornix and possibly through to the posterior of the capsule. Furthermore, posterior capsule tears facilitate the nucleus being “dropped” into the posterior chamber resulting in further complications.
Further problems that may develop in capsulorhexis are related to inability of the surgeon to adequately visualize the capsule due to lack of red reflex, to grasp it with sufficient security, to tear a smooth symmetric circular opening of the appropriate size or technical difficulties related to maintenance of the anterior chamber depth after initial opening, small size of the pupil or the absence of a red reflex due to the lens opacity. Additional complications arise in older patients with weak zonules and very young children that have very soft and elastic capsules, which are very difficult to mechanically rupture.
Errant tears and other problems can be managed to some degree with the addition of dense viscoelastics and relaxing incisions to assist in stabilization of the capsule rim. If need be a two-stage capsulorhexis technique can be required to rescue a situation or for the formation of a second large capsulorhexis. Specifically, after creating an initial capsulorhexis, the anterior cortex is removed with aspiration and irrigation and the capsule is refilled with viscoelastic to stabilize the capsule and the capsulorhexis is enlarged by tearing a larger second capsulorhexis.
Further stress is applied to the capsulorhexis in phacoemulsification and implantation of the IOL, if the capsulorhexis is discontinuous or if the force is too great the capsulorhexis may cause a radial tear. This is dealt with as above. Furthermore if the posterior capsule integrity is damaged during the cataract surgery (i.e., a posterior capsule rent) dense viscoelastic is added to preserve its shape and a posterior capsulorhexis is performed, to salvage the integrity and strength of the capsule around its equator such that an IOL can be implanted.
Most current cataract surgeries are performed with the aid of phacoemulsification and typically include the following eighteen steps.
1. Pupil dilation
2. Local anesthesia
3. Placement of the lid speculum
4. Entry into the eye through a small incision (typically 2-3 mm in length, at the edge of the cornea)
5. Viscoelastic injected into the anterior chamber to maintain the volume and shape of the anterior chamber and cornea during the rest of the procedure
6. Zonule function test
7. Capsulorhexis
8. Hydrodissection intended to identify and soften the nucleus for the purposes of removal from the eye.
9. Hydro-delineation to increase the distance between the nucleus and the posterior capsule, thus providing a safety zone.
10. Nuclear cracking or chopping if needed
11. Ultrasonic with aspiration and irrigation used to sculpt and emulsification of the nucleus used to sculpt the relatively hard nucleus of the lens.
12. Aspiration of the residual soft cortex
13. Capsular polishing
14. Once the capsule is empty further addition of viscoelastic to maintain the volume and shape of the capsule
15. Implantation of the artificial IOL
16. Centration of the IOL
17. Viscoelastic removal
18. Wound sealing/hydration (if needed)
Following cataract surgery there is a rapid 1-2 days response where the capsule hardens and the capsule contraction starts. This continues over a 4-6 week period where fibrosis of the capsulorhexis and IOL optic interface, and the haptic and capsule interfaces also occur. Even beyond one year the capsule continues to contract to a lesser degree, thus positioning the capsulorhexis is a critical factor in the long-term success.
Following cataract surgery one of the principal sources of visual morbidity is the slow development of opacities in the posterior lens capsule, which is generally left intact during cataract surgery as a method of support for the lens, and also as a means of preventing subluxation posteriorly into the vitreous cavity. It has been estimated that the complication of posterior lens capsule opacification occurs within 5 years of surgery with a 10% incidence. This problem is thought to occur as a result of epithelial and fibrous metaplasia along the posterior lens capsule centrally from small islands of residual epithelial cells left in place near the capsule equator. Q-switched Nd:YAG lasers are utilized to perform a noninvasive posterior capsulotomy to remove the opacification from the proximity of the optical axis.
Accordingly, there is a need in the art to provide new ophthalmic methods, techniques and apparatus to advance the standard of care for capsulorhexis.